Generator of high current pulses

ABSTRACT

A circuit for pulsing the output of a dc power supply (2) connected across a load such as a welding arc. The circuit has a first SCR (12) connected in parallel with a resistor (14). The SCR (12) controls the pulse current and the resistor (14) controls the background current. Each pulse is terminated by the action of a second SCR (18) and a capacitor (16) connected across the first SCR (12) by applying a control pulse to the gate of a second SCR (18). The capacitor (16) is charged by a power supply (20) and is connected by the second SCR (18) so that the first SCR (12) is reverse biased and hence is turned off to terminate the pulse. Immediately the first SCR (12) turns off, the capacitor (16) is charged in the opposite sense to the charge derived from the power supply (20) and this in turn causes the second SCR (18) to be reversed biased by conduction through the resistor ( 14) and turned off a short time after the first SCR (12) is turned off. The circuit permits control of individual pulses and pulse trains in accordance with variations in any of a range of external control variables.

This application is a continuation of application Ser. No. 261,208 filedMay 6, 1981, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a circuit for producing large pulses ofelectric current especially useful in welding.

Pulsed electric currents of high value are used in many applications, aparticularly important one is in welding including spot welding. Otherimportant applications include pulsed electroplating, D.C. to D.C.inverters, D.C. electric motor speed controls and surface hardening bypulsed arcs. In these applications an approximately square wave currentpulse is desired and current pulses of up to 10,000 amp may be required.It is for such large current pulses that silicon control rectifiers((SCR's) are most appropriate as solid state switching devices.

The preferred embodiment of the invention to be described uses SCR's asswitching elements in its circuit which is used to control the output ofa dc power supply of conventional nature and the one pulse unit can beused with a wide range of power supplies such as motor generator sets,lead-acid storage batteries and transformer-rectifier units. In thisfashion the cost of installing pulsed welding facilities, for example,is significantly reduced because continuous current welding powersupplies which have already been installed, can be used with the pulsingunit.

LAWRENCE, B. and JACKSON, C. in 1969 Variable Frequency Gas ShieldedPulse Current Arc Welding. Welding Journal Res. Suppl. 48 1969,97s-104s. reported a study of pulsed current arc welding in which theinfluence of the pulse current frequency, duration and magnitude on themode of metal transfer in the MIG (metal-inert gas) arc was determined.This experimental equipment used SCR's switching the output of a dcpower supply with the control pulses to the gates of the SCR's beingobtained from a mechanical commutator. In order to maintain a continuousstanding current so that the arc did not go out at the end of thecurrent pulse a second power supply was used. The use of a mechanicalcommutator and multiple power supplies would not now be seen as elegantdesign. Furthermore the switching circuit which used two SCR'sfunctioned in a bistable mode of operation in which each SCR was used toturn the other off alternately. The larger SCR controlled the arc pulsecurrent and the smaller SCR conducted for the time that the arc currentpulse was not flowing. This method of operation is not only inefficientbut also enhances the problem of disposal of surplus heat produced bythe pulse generator.

Two other pulse shaping units for welding have been described whichswitch the output of a welding power supply, SMITH G. A. and BROWN, M.J. An Inverter Power Source for Welding Applications. 2nd InternationalConference on Power Electronics--Power Semiconductors and theirApplications. London England: I.E.E. 1977 58-61, and LOWERY, J. A NewConcept for AC/DC Power Sources for TIG-Welding. Advances in WeldingProcesses 4th International Conference Harrogate England, 4th-11th May1978, 161-169. These designs have been developed specifically for TIG(tungsten-inert gas) welding of aluminium for which application it ishighly advantageous to have the current through the arc alternating thusproducing a cleaning action on the workpiece surface by ion bombardmentwhen it is biased negative with respect to the tungsten electrode. Eachpulse unit is of fixed frequency (50 or 60 Hz). The cleaning effect aswell as the weld penetration and heat input to the workpiece are changed(but not independently) by varying the ratio of the durations of thepositive and negative half cycles of current.

Another device described by GRIST, F. J., Improved, Lower Cost AluminiumWelding with Solid State Power Source. Welding Journal. 54 1975,348-357, is generally similar to those mentioned immediately above butoffers in addition limited control of the frequency of operation (50-200Hz). This frequency limitation is imposed by the use of a resonantcommutation circuit for the SCR's.

One of the shortcomings of the above and other known circuits is thatthe control of the switching of the current pulses may be dependent onparameters which do not reflect the conditions at the arc itself.

The object of the present invention is to provide a circuit forgenerating current pulses of up to at least 10,000 amps in which theswitching of the current pulses is controlled in accordance with thevoltage at the load.

SHORT STATEMENT OF THE INVENTION

According to the present invention, there is provided a circuit forgenerating current pulses in a dc power supply (2) comprising aswitchable element (12) connected to said power supply (2) and to aload, control means for controlling the state of said switchable element(12) whereby a relatively heavy current pulse is allowed to flow throughthe load while said switchable element (12) is in its on state, andmeans (14) for supplying a relatively light background current to theload while said switchable element (12) is in its off state, saidcontrol means including trigger means (82, 86) for generating controlsignals for controlling the state of said switchable element (12) and avoltage sensing circuit (80, 84) for sensing the voltage across saidload and causing said trigger means (82, 86) to generate a controlsignal to switch said switchable element (12) to its on state when saidsensed voltage is at a predetermined low level and to switch saidswitchable element (12) to its off state when said sensed voltagereaches a predetermined high level.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described with reference to theaccompanying drawings, in which:

FIG. 1 is a simplified circuit diagram of the pulse generator of theinvention;

FIG. 2 is a waveform diagram useful in understanding the operation ofthe circuit of FIG. 1;

FIG. 3 illustrates a modified circuit; and

FIGS. 4 and 5 illustrate control circuitry for generating gate pulses.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The circuit illustrated in FIG. 1 is for use in pulsed weldingoperations, the circuit showing a conventional welding current supplyapparatus 2 having its positive output connected to a work piece 4 andits negative output connected to an electrode 6 through the pulsecircuit 8 of the invention. The circuit 8 includes a high frequencystarting circuit 10 for initiating an arc between the electrode 6 andthe work piece 4. The circuit includes a primary SCR 12 which is oflarge capacity and carries the full arc current, when the arc currentflows. A resistor 14 is connected across the primary SCR 12 so as toprovide a standing arc current in the range say 20 to 60 amp at thosetimes when the remainder of the circuit would not permit current flow.The value of resistor 14 would typically be 0.5 ohm. The circuit furtherincludes a capacitor 16 and secondary SCR 18 which are connected in abranch across the SCR 12. The circuit also includes a secondary powersupply 20, the positive output of which is connected to the junction ofthe capacitor 16 and secondary SCR 18, the main function of thesecondary power supply 20 being to supply charging current to thecapacitor 16.

The operation of the circuit will now be described with reference toFIG. 2. The waveforms 13, 19 and 17 represent the voltages across SCR12, SCR 18 and capacitor 16 respectively, and the waveform 19 representsthe arc current flowing between the workpiece 4 and the electrode 6.

When the power supplies are switched on, the SCR's 12 and 18 are notconducting and the capacitor 16 is charged by power supply 20 throughresistor 14, and SCR 18 is biased in the forward direction (≃150 volt).When the arc is initiated by the HF starting circuit 10, the currentthrough it is controlled by the value of resistor 14. At the same timethe potential of the point 22 in the circuit with respect to the commonpoint 22 rises because of the voltage drop across resistor 14 due to thearc current passing through it. The circuit is then in the conditionindicated at time t₁ in FIG. 2.

A control pulse generated in a manner to be described in greater detailbelow is applied to the gate 26 of SCR 12 at time t₂ to initiateconduction and the voltage across it falls to about 1 volt so thatnearly all the arc current passes through it. The arc current rises to anew value determined essentially by the electrical characteristics ofthe supply apparatus 2 and the arc voltage. The rapidly falling voltageacross SCR 12 causes a rapid drop in the voltage across SCR 18 followedby a slower rise as power supply 20 charges capacitor 16. The rate ofrise of the arc current is limited by the impedance of supply apparatus2.

The arc current pulse is terminated at time t₃ by applying a controlpulse to the gate 28 of SCR 18. When SCR 18 conducts, the capacitor 16is connected across SCR 12 in a low impedance circuit causing SCR 12 tobe immediately turned off by the reverse bias due to the capacitor 16.At the same time the full arc current flows to the capacitor fordischarging it and then reversing its polarity. The voltage across thearc is momentarily increased producing a spike in the arc currentfollowed by a rapid fall to below the standing current level with asubsequent return to that level.

At the same time the capacitor 16 is connected across SCR 18 through aresistor 14 tending to reverse bias SCR 18. This tendency is opposed bythe power supply 20. If the impedance of the power supply 20 issufficiently high (and this condition is almost automatically guaranteedbecause the supply has to be able to withstand the near short circuitcondition imposed by SCR 18) the influence of the reverse bias of thecapacitor prevails and SCR 18 is turned off. Between times t₃ and t₄ SCR18 is still forward biased at 0.7 volt because its reverse recoverycurrent inhibits the reverse bias. At time t₄ its recovered chargerequirement is satisfied allowing the bias on SCR 18 to become negativeuntil recharging of capacitor 16 by the power supply 20 becomes thedominant process and then the voltages on SCR's 12 and 18 and capacitor16 return to the values at time t₁.

A prototype of this circuit has been tested and found to perform in amost satisfactory manner. In the prototype arrangement the value of thecapacitor 16 was 525 μF and the value of the resistor 14 was 0.5 ohm.The voltage output of power supply 20 is approximately 110 volts.Generally speaking, the higher the value of the voltage of supply 20 thesmaller the value of capacitor 16. Of course the smaller the output ofsupply 20 the smaller is the current drain therefrom.

A preferred addition to the circuit is indicated in the schematiccircuit of FIG. 3 where the components SCR 30, resistor 32 and switch 34are added to the branch of the circuit containing resistor 14. With theswitch 34 open SCR 30 does not conduct and consequently the standingcurrent can not flow through resistor 14. With switch 34 closed SCR 30always conducts current when SCR 12 is off so that the standing current,limited by the value of resistor 14 is able to pass. The value ofresistor 32 is chosen so that the current flowing to the gate of SCR 30is always within its specified range. The function of the circuitaddition is to allow the main power supply to be left on and yet have novoltage on the output terminals provided power supply 20 is inactivated.This is important in welding applications where switch 34 can be on thewelding head to improve operator safety and convenience. The circuitwill operate cyclically in this manner provided appropriate pulses areapplied to the gates 26 and 28 of the SCR's, and the generation of suchpulses will be described with reference to FIGS. 4 and 5.

FIG. 4 illustrates a suitable control circuit for generating gate pulsesfor the gates 26 and 28 of the SCR's 12 and 18 which may if desired beoperated in a manner which is not preferred, independently of thevoltage of the arc. The circuit basically comprises four monostablecircuits (NE555 integrated circuits) 36, 38, 40 and 42. When a switch 44is closed the monostable circuit 36 will produce a pulse at its output46 after a predetermined period. The pulse at the output 46 will turn ontransistors 48 and 50 which will in turn produce a suitable pulse at thepoint 52 which is connected to the gate 26 of the SCR 12 and so turnthat SCR on. Output from the monostable circuit 36 is applied to theinput of the second monostable circuit 38 and after a predetermineddelay depending on the setting of switch 54, the circuit 38 will producean output pulse at its output 56 which is applied to the input of thethird monostable circuit 40. After a predetermined delay, the circuit 40will produce a pulse on its output 60 which turns on transistors 62 and64 thereby producing a pulse at the point 66 which is connected to thegate 28 of the SCR 18. This corresponds to the time t₃ of FIG. 2 and thecapacitor 16 will be discharged thereby turning SCR 12 off. The outputof the monostable circuit 40 is applied to the input of the fourthmonostable circuit 42, the output 68 of which is fed back via a switch70 to the input of the first monostable circuit 36 so that if the switch70 is closed the circuit will cyclically produce gate pulses for theSCR's 12 and 18.

The above described operation of the control circuit shown in FIG. 4 ismodified in accordance with the present invention so that it does notcycle in a selective period but is triggered in response to an externalsource. One particularly convenient source for such triggering signalswould be the magnitude of the arc voltage. To effect this selectorswitches 72 and 74 are connected to the inputs of the first and thirdmonostable circuits 36 and 40. If the selector switches 72 and 74 aremoved so as to connect with terminals 76 and 78, externals pulsesapplied to those terminals can be used for triggering arc current pulseoff or arc current pulse on respectively.

FIG. 5 shows a simplified circuit for generating control signals whichare responsive to the arc voltage i.e. the voltage between the electrode6 and the workpiece 4. The circuit includes a first opto-coupler 80 theinput of which is directly connected across the electrode 6 andworkpiece 4, the output being connected to a first input of a comparator82. A selectable reference level is applied to the other input of thecomparator and the output is connected to the terminal 78 shown in FIG.4. The circuit includes a second opto-coupler 84 connected to a secondcomparator 86 the other input of which has applied thereto a secondreference level and the output of the comparator 86 is connected to theterminal 76 shown in FIG. 2. A particularly useful arrangement for thecircuit shown in FIG. 5 would be to set the comparator 82 so that itproduces an appropriate pulse at its output when the arc voltage reachesa predetermined level. The second comparator 86 would be set so that itwould produce an appropriate output when the arc voltage falls below apredetermined value. The generation of such signals can provide veryuseful controls for welding operations.

Returning to FIG. 3, that part of the circuit illustrates a furthermodification which is designed to reduce the possibility of unwantedtriggering of SCR 12 due to stray pulses which are sometimes generatedwhen other SCR's in a circuit are themselves triggered. In thisarrangement a phase shifting element namely capacitor 88 is connected inseries with the gate electrode 26. The circuit also includes a steeringdiode 90 in series with the secondary of a pulse transformer 92 whichreceives appropriate pulses from the point 52 of the control circuitshown in FIG. 4. It is necessary to include a resistor 94 to preventcharging of the capacitor 88 with consequent reduction of pulseamplitudes being delivered to the gate electrode 26.

The principal advantages of the preferred embodiment of the inventionare as follows:

1. A separate power supply 20 is used to charge the commutatingcapacitor 16. This permits a voltage higher than the arc voltage to beused thus ensuring that switching continues reliably even if the arc isvery short or short circuited. In addition, the capacitance of capacitor16 can be reduced progressively as the voltage of supply 20 isincreased.

2. The power supply 20 in a preferred embodiment comprises a slightlyreactive transformer and a full wave rectifier. This prevents excessivecurrent drain when SCR 18 is on and permits another monostable mode ofoperation as described in the next section. It need only have a nominalrating of about 500 watts, which is much lower than prior art devices.

3. SCR 12 is bypassed by the resistor 14 which also has a twofoldfunction. Firstly it eliminates the need for a second power supply asused in known SCR pulse units connected to the output of the powersupply. Secondly, it is vital to allow this unit to operate as amonostable rather than a bistable device. This reduces the power whichhas to be dissipated in the unit during operation. If resistor 14 isomitted another mode of operation is possible, this requires that thepower supply 20 be unfiltered (with resistor 14 present in the circuit,the capacitor 16 acts as a filter condenser for the supply). In thismode of operation after SCR 18 is triggered on, it will remain on untilthe next current zero of the power supply when, if it is sufficientlyfast, it will turn off. This method imposes a variable delay before SCR18 can turn off after commutating off SCR 12 and consequently limits themaximum repetition rate of the switching circuit. At a sufficiently highrepetition rate SCR 12 is triggered on before the next current zero ofpower supply 20; the switching is then similar to the mode used in thepulse unit devised by LAWRENCE, B. and JACKSON, C.

4. The arc voltage is monitored by the machine and the control iscapable of switching the pulse current on at one selected value and offagain at another selected value of arc voltage where (in general) thelatter voltage is higher than the former.

There are two alternate modes of operation and both of these depend onthe use of an electrode wire which is being fed at constant speedtowards the weld pool.

These modes are:

(a) the pulse current is switched on at a selected voltage and thelength of the pulse is controlled by a timer,

(b) the pulse current is switched on at a selected arc voltage andswitched off again at a selected higher voltage.

Mode (a) is appropriate for extending the range of the dip transferprocess. The dip transfer or "short arc" welding procedure isparticularly appropriate to out of position (e.g. vertical and overhead)welding and for the arc welding of thin sheet. In this mode the sensingof the contact of the electrode end to the weld pool then initiates apulse of current which lasts for a predetermined time. This enables morepower to be delivered to workpiece (by increasing the length of thepulse) and so inhibit the greatest problem of the dip transferprocess--namely its tendency to cause lack of fusion defects.

Mode (b) is suitable for MIG and submerged arc processes. Another usefulapplication of mode (b) would be for welding, particularly aluminium,whereas spatter increases finishing costs.

The small spatter drops usually are emitted by an explosive disruptionof the bridge between a departing liquid metal drop and the electrode.It is thought that the problem can be controlled by reducing the arccurrent to a low value during the separation of the drop from theelectrode.

We claim:
 1. A circuit for generating by a power supply (2) arc currentpulses for welding comprising:a workpiece (4); an electrode (6); andmeans for establishing an arc across said workpiece (4) and saidelectrode (6), said arc establishing means including a switchableelement (12) connected to said power supply (2) and to said electrode(6), and control means for controlling the state of said switchableelement (12) to maintain said current pulse, a relatively heavy currentpulse being allowed to flow through the arc while said switchableelement (12) is in its on state, and a relatively low current beingallowed to flow through the arc while said switchable element (12) is inits off state, said control means including a trigger means (82, 86) forgenerating first and second control signals for controlling the state ofsaid switchable element (12) and a voltage sensing circuit (80, 84)connected to sense a voltage representing a parameter across the arc,said voltage sensing circuit (80, 84) being connected to said triggermeans (82, 86) to cause said trigger means (82, 86) to generate a firstcontrol signal to switch said switchable element (12) to its on statewhen said sensed voltage is at a predetermined low level and generate asecond control signal to switch said switchable element (12) to its offstate when said sensed voltage reaches a predetermined high level.
 2. Acircuit as claimed in claim 1, wherein said voltage sensing circuit (80,84) includes a comparator circuit (82, 86) for generating said controlsignals, said comparator circuit including means for applyingpredetermined reference voltage levels to said comparator circuitwhereby said control signals are generated when said predeterminedvoltage levels are detected.
 3. A circuit as claimed in claim 1 or 2,wherein said circuit comprises first and second parallel branches, saidfirst branch including an impedance (14) having a predeterminedresistive value, said second branch having a low impedance pathincluding said switchable element (12) whereby when said switchableelement (12) is in its off state, said relatively light backgroundcurrent flows through said first branch and said load.
 4. A circuit asclaimed in claim 3, wherein said switchable element comprises a firstSCR (12) connected in parallel with said impedance (14), said circuitfurther comprising a capacitor (16) and a second SCR (18) connected in abranch across said first SCR (12), said first control signal beingapplied to the gate of said first SCR (12) to switch said first SCR (12)to its on state, said second control signal being applied to the gate ofsaid second SCR (18) to switch said second SCR (18) to its on state andto connect said capacitor (16) across said first SCR (12) whereby saidfirst SCR (12) is in use switched to its off state by the reverse biasapplied by the charge stored in the capacitor (16).
 5. A circuit asclaimed in claim 4, further comprising a secondary power supply (20)connected to said capacitor (16) to supply charging current to saidcapacitor (16), said power supply (20) having a high output impedance.6. A circuit as claimed in claim 4, wherein said capacitor (16) isconnected to be reverse charged by the current flowing to the load whensaid first SCR (12) is turned to its off state, said capacitor (16)reverse biasing said second SCR (18) to turn said second SCR (18) to itsoff state.
 7. A circuit as claimed in claim 1 or 2, wherein saidswitchable element comprises a first SCR (12) connected in parallel withan impedance (14), said circuit further comprising a capacitor (16) anda second SCR (18) connected in a branch across said first SCR (12), saidfirst control signal being applied to the gate of said first SCR (12) toswitch said first SCR (12) to its on state, said second control signalbeing applied to the gate of said second SCR (18) to switch said secondSCR (18) to its on state and to connect said capacitor (16) across saidfirst SCR (12) whereby said first SCR (12) is in use switched to its offstate by the reverse bias applied by the charge stored in the capacitor(16).
 8. A circuit as claimed in claim 7, further comprising a secondarypower supply (20) connected to said capacitor (16) to supply chargingcurrent to said capacitor (16), said power supply (20) having a highoutput impedance.
 9. A circuit as claimed in claim 7, wherein saidcapacitor (16) is connected to be reverse charged by the current flowingto the load when said first SCR (12) is turned to its off state, saidcapacitor (16) reverse biasing said second SCR (18) to turn said secondSCR (18) to its off state.
 10. A circuit as claimed in claim 1 or 2,wherein said power supply (2) comprises welding apparatus and said loadcomprises an arc struck between an output electrode (6) of the weldingapparatus and a workpiece (4).